You see,
I don't think that the devs are abandoning the idea
*3 years later*
Dev:Hey remember penumbra
Dev 2:Yes
Dev: Wanna make another one
Dev 2: Hell yeah
Rest of devs:What another Penumbra?
YAYYYYYYY
*a few devs faint*

Anyway back to today,they are not abandoning the idea,simply adding it to an archive for later use,
a company can not live on one game series alone.
They will try another idea,possibly using the same engine,
and simply come back to Penumbra later.
For those of you fanboys who want it NOW:
Just get involved in mods